I do not own a Glock 35, but, I own a Glock 34. It is the 9MM ver. of the 35, same thing. I also own a H&K USP .40. That said here is what I think.
H&K parts can be hard to come by and there customer service SUCKS. Even a recoil spring can be tricky to track down at a local gunshop. They truly do not care about there civilian market, they really only care about there influence in the military. It showes in there price tag.(I did not pay for mine, it was a gift) So, stuff like trigger jobs, sights, custom springs and even extra mags. are mostly out of the question and will cost and arm and a leg to get and may take months. I have seen send backs go to H&K for repairs and warranty work take up to 6 months. On the plus side, most have good triggers to start out with, good sights too. They are really pretty good shooters. But, IMHO I would go with SIG befor a H&K if I was going to spend close to 800 bucks on a DA/SA pistol with a tack rail.
As for Glock it is tottaly opposite of H&K. Good customer service and parts our every where. Pretty cheep too. Most local gunshops have on hand or can get with in a day, mags, 20 different sights to pick from, custom recoil srings and guide rods, 3 1/2 pound trigger jobs, extended mag and slide releases, just to name a few. As for the 35 it already comes with some nice goodies. The 35 is there .40cal. traget pistol so you get some cool stuff with it like, extended slide and mag. release, ajust rear sight and 3 1/2 pound trigger all come stock on the 35 just like the 34, plus the slide has been cut out on the top for weight.
Good luck.
Dave.